This invention relates to a welding power unit of the type which is adapted to charge a capacitor for producing a DC welding current and, more particularly to a control circuit of the power unit to improve the charging characteristics of the power unit.
A capacitor type welding power unit is usually used in a resistance welding machine. Such a power unit includes one or more capacitors that by degrees store a predetermined amount of electrical energy necessary for the desired welding, and instantaneously discharge most of the stored electric energy to a welding transformer to produce a DC welding current for the welding. The advantage of the power unit of this type is that the supply voltage can be reduced and the size of the circuit can be reduced.
FIG. 10 shows a circuit diagram of a typical prior art capacitor type welding power unit. The power unit inputs a commercial AC voltage Eo of a predetermined value, for example 100 V through a pair of input terminals 102a and 102b. The AC voltage Eo is applied to the primary winding of a charging transformer 104 which is a single-phase transformer for boosting the AC voltage Eo to obtain an AC voltage E1 of a predetermined value, for example 400 V on the secondary winding thereof. The AC voltage E1 is applied to a rectifier circuit 108 only when either a thyristor 106a or a thyristor 106b is in the "ON" or conductive state.
When the thyristor 106a is conductive in the positive half-cycle of the AC voltage E1, a current Ic flows in a closed circuit established by the secondary winding of the transformer 104, diodes 108a and 108c of the rectifier circuit 108, a resistor 110, a pair of capacitors 112 and 114, and the thyristor 106a. When the thyristor 106b is conductive in the negative half-cycle of the AC voltage E1, the current Ic flows in another closed circuit established by the secondary winding of the transformer 104, diodes 108b and 108d of the rectifier circuit 108, the resistor 110, the capacitors 112 and 114, and the thyristor 106b. Each time the current Ic flows, the capacitors 112 and 114 are charged with some amount of electrical charge.
In this manner, the thyristors 106a and 106b are alternately turned on at a predetermined control angle or angle of retardation in each half-cycle of the commercial AC frequency, and the resulting current Ic, having an average or RMS value in accordance with the control angle, is supplied as a charging current to the capacitors 112 and 114. The thyristors 106a and 106b are respectively turned on by thyristor firing pulses Ga and Gb which are provided from a charging control circuit 130.
The charging voltage Ec of the capacitors 112 and 114 are monitored by the charging control circuit 130. When the charging voltage Ec reaches a predetermined value Eco, the circuit 130 stops the firing of the thyristors 106a and 106b to complete the charging of the condensers 112 and 114.
About the time when the charging is completed, a welding start circuit 170 turns a thyristor 122 on so that a discharging circuit for the capacitors 112 and 114 is established by the thyristor 122 and the primary winding of a welding transformer 120. Thus, the condensers 112 and 114 instantaneously discharge their electrical charge, and thereby a current Id flows in the discharging circuit and a corresponding large current Iw flows as a welding current in the secondary circuit of the welding transformer 120 which includes a pair of electrodes 124a and 124b, and workpieces 126 and 128 to be spot-welded.
FIG. 11 shows the circuit diagram of a conventional charging control circuit 130, and FIG. 12 shows various waveforms of voltage signals and the current appearing in the circuit 130 as well as the waveform of the charging current Ic.
Referring to FIG. 11, a PUT (Programmable Unijunction Transistor) 146, a capacitor 148 and resistors 150 and 156 constitute an oscillation circuit to generate a current pulse ip with a fixed period which, typically, is shown in FIG. 12(E). The current pulse ip periodically makes a transistor 160 conductive so that the thyristor firing pulses Ga and Gb are generated from the secondary winding of a transformer 162, thereby obtaining the charging current Ic which is shown in FIG. 12(F). A clock circuit 138 provides a clock pulse CK of a frequency which is the same as to that of the power source to one of the input terminals of an OR gate 140. When the output voltage CO of a comparator 136 is at an "L", i.e. low, level the clock pulse CK can pass through the OR gate 140 and makes a transistor 144 conductive, thereby forcing the capacitor 148 of the PUT circuit to discharge its electrical charge. Each pulse of the current ip is generated after the passage of a time period .theta.o from the beginning of each discharging. The time period .theta.o is determined by the capacitance of the capacitor 148 and the resistance of the resistor 150. Thus, the time period .theta.o is fixed during the charging of the capacitors 112 and 114 and, accordingly, the control angle of the thyristors 106a and 106b is also fixed.
A charging voltage setting circuit 132 provides a constant voltage S[Eco] representing a target value Eco of charging voltage to one of the input terminals of a comparator 136, and a charging voltage detection circuit 134 provides a voltage signal S[Ec] representing the momentary charging voltage Ec of the capacitors 112 and 114 to the other input terminal of the comparator 136. When the capacitors 112 and 114 are charged, the charging voltage Ec increases along a logarithmic curve to the target value Eco as shown in FIG. 12(G). When the target value Eco is attained, the output voltage of the comparator 136 turns an "H", i.e. high, level (see FIG. 12(A)), and hence the OR gate 140 shuts off the clock pulse CK (see FIG. 12(C), making the PUT circuit stop its oscillation (see FIGS. 12(D) and 12(E)). As the result, the generation of the thyristor firing pulses Ga and Gb terminates and, accordingly, the supply of the charging current Ic to the capacitors 112 and 114 is completed (see FIG. 12(F)).
However, the power unit including the conventional charging control unit described as above has several disadvantages as follows.
First, when the supply voltage Eo fluctuates, the charging time required for the charging voltage Eo to reach the target value Eco varies, as shown in FIG. 13. In the case where the charging time exceeds a preset charging time, there is a danger of the welding operation starting before the charging voltage reaches the target value Eco because the welding start circuit 170 is set to generate a welding start signal or trigger signal for firing the thyristor 122 in the discharging circuit soon after the passage of a preset charging time. Such a premature starting of welding may be avoided by delaying the time of generation of the welding start signal. But, delaying of a welding start time elongates the duration of each welding cycle, leading to a lowering of the welding productivity.
Secondly, it is necessary to adjust the time constant .theta.o of the PUT circuit of the charging control circuit 130 according to the frequency of the power source. When the frequency of power source is changed from 60 Hz to 50 Hz, for instance, the necessary charging time is prolonged when the time constant .theta.o is not adjusted. To maintain the necessary charging time unchanged, it is necessary to reduce the time constant .theta.o so as to expand or increase the width or the RMS value of the pulses Ic(1), Ic(2),--of the charging current Ic. Thus, the resistor 150 determining the time constant .theta.o of the PUT circuit must be replaced by another resistor having a different resistance manually or by providing a particular change-over switch.
Thirdly, when the number of the capacitors is increased or decreased to change the welding power, the charging resistor 110 must be replaced by another resistor having a different resistance. If a capacitor is added in parallel to the capacitors 112 and 114 to increase the welding power, for instance, the time constant of the charging circuit is increased and, accordingly, the necessary charging time would be lengthened when the same resistor 110 is used. To ensure that the necessary charging time is maintained constant, the resistor 110 must be replaced by a resistor having a smaller resistance.
Fourthly, the peak level of the charging current Ic is maximum immediately after starting of charging and then decreases exponentially as shown in FIG. 12(F). Such charging characteristics cause the generation of a large amount of heat or power loss in the resistor 110, thereby decreasing charging efficiency.
Fifthly, when the target value Eco of the charging voltage is set far less than the allowable voltage of the capacitor, the charging voltage Ec rises near the target value Eco with the supply of only a few charging current pulses Ic(1), Ic(2),--which have the highest peak levels or largest RMS values as described above. In that case, the charging voltage Ec is likely to fail to precisely reach the target value Eco whereby the weld quality may be lowered.
Lastly, there is a further problem derived from the charging resistor 110 which is connected between the rectifier circuit 108 and the capacitors 112 and 114 for restricting the charging current Ic which flows into the charging circuit. Since the charging current Ic of a high value is supplied through the charging resistor 110 to the capacitors 112 and 114 to store a large amount of electrical energy for a welding operation, a high resistance heat is generated in the resistor 110. This is not only undesirable in respect of the efficiency of power consumption, but also needs a cooling installation, particularly, a water cooling system in the case of a large-scaled welding power unit. Moreover, the resistor 110 is apt to be fatigued with its resistance heat and it is necessary to replace the resistor 110 after a relatively short time period of usage.